1. Field of the Invention
The present invention pertains to a wave front interferometer which can determine the surface shape of an optical component.
2. Description of the Prior Art
An apparatus is known that utilizes optical interference to determine the shape of the optical surface of some object such as, an optical part. This conventional apparatus is described in Japanese Patent Publication No. H3-21806.
FIG. 17 illustrates the conventional interferometer disclosed in the above Publication. Light emitted from a laser light source (1) is focused through a condenser lens (2), and then diverted and reflected at a beam splitter (3). After forming a parallel beam through a collimator lens (4) and then forming a focusing beam through a condenser lens (5), the light beam is incident on an optical surface (12) of the lens to be examined (11) that is secured by a lens holder (10). The beam reflected on the surface (12) interferes with a beam reflected by the reference surface (5A) of the condenser lens (5), yielding interference fringes. The interference fringes are then observed using an observation mechanism (6), such as a telecamera, to determine the shape of the surface (12). However, the conventional interferometer has the problems described below.
In the construction of the conventional apparatus shown in FIG. 17, the laser beam needs to be vertically incident on the surface (12). It is also necessary to keep the optical system in a position relative to the laser light source (1) and the surface (12) such that an optical path is formed such that the condenser lens (5) may cause the laser beam to focus on a centripetal position (13) of the surface (12). FIGS. 18 and 19 illustrate such positional relationships for a concave and a convex surface, respectively.
As shown in FIG. 18, for a concave surface, the centripetal position (13) of the surface (12) is positioned between the optical system and the surface (12), thereby the distance between the condenser lens (5) and a surface (12) is greater than the radius of curvature of the surface (12). Consequently shape-determination becomes possible.
On the other hand, for a convex surface, as shown in FIG. 19, the centripetal position (13) of an optical surface (12) is outside of the optical system. In other words, beyond the surface (12). When the distance (L) between the reference surface (5A) of the condenser lens (5) and the focus position of the laser beam is smaller than the radius of curvature of the surface (12), there is no way to focus the laser beam on the centripetal position (13) of the surface (12). This makes shape determination impossible for a convex surface with a radius of curvature larger than the distance (L).